The present invention relates to radiation tolerant metal oxide semiconductor field effect transistors (MOSFET""s) and more particularly to a method for compensating for the ionizing radiation induced threshold shift in MOSFET""s within MOS integrated circuits.
The MOS device is the heart of present day electronics and MOSFET""s are widely used in commercial space applications where they are exposed to radiation. The harsh radiation causes a build up of trapped electric charge in the oxide, which causes a shift in the transistor gate threshold voltage, resulting in a failure of the device.
Radiation tolerant MOSFET""s have been proposed to address this problem. However, the process used to fabricate radiation tolerant integrated circuits (IC""s) differs substantially from standard fabrication processes. As a result of the highly specialized fabrication process, radiation tolerant IC""s are extremely costly.
The present invention is a method for making a radiation tolerant MOSFET using standard MOSFET integrated devices. The method offsets the threshold shift by way of a compensation circuit built into the design of the MOS integrated circuit. The gate threshold voltage has been found to be a function of the body threshold voltage. The compensation circuit adjusts the body voltage of a transistor in the active circuit by using the threshold shift of other transistors in the compensation circuit, on the same integrated circuit.
The radiation induced threshold shift in a first transistor is compensated by using the radiation induced threshold shift in a second transistor, or a network of transistors, to shift the body voltage of the first transistor, thereby significantly improving the radiation tolerance of the first transistor.
The method of the present invention produces a radiation tolerant MOS device that can be produced using any available commercial process, without the need for special fabrication. The result is a significant cost reduction for radiation tolerant MOS IC""s because they can be produced using IC fabrication processes that are readily available.
It is an object of the present invention to improve the radiation tolerance of MOSFET""s. It is another object of the present invention to produce a radiation tolerant MOSFET without the need for highly specialized, expensive fabrication.
It is a further object of the present invention to provide a compensation circuit to offset the threshold voltage shift induced by radiation. It is yet a further object of the present invention to design the compensation circuit based on the gate threshold voltage as a function of the body voltage of the MOSFET.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.